Cesare Chiosi, Emiliano Merlin, Lorenzo Piovan
Early-type galaxies obey a narrow relation traced by their stellar content between the mass and size (Mass-Radius relation). The wealth of recently acquired observational data essentially confirms the classical relations found by Burstein, Bender, Faber, and Nolthenius, i.e. log R_1/2 \propto log M_s^0.54 for high mass galaxies and log R_1/2 \propto log M_s^0.3 for dwarf systems (shallower slope), where R_1/2 and M_s are the half-light radius and total mass in stars, respectively. Why do galaxies follow these characteristic trends? What can they tell us about the process of galaxy formation? We investigate the mechanisms which concur to shape the Mass-Radius relation, in order to cast light on the physical origin of its slope, its tightness, and its zero point. We perform a theoretical analysis, and couple it with the results of numerical hydrodynamical (NB-TSPH) simulations of galaxy formation, and with a simulation of the Mass-Radius plane itself. We propose a novel interpretation of the Mass-Radius relation, which we claim to be the result of two complementary mechanisms: on one hand, the result of local physical processes, which fixes the ratio between masses and radii of individual objects; on the other hand, the action of cosmological global, statistical principles, which shape the distribution of objects in the plane. We reproduce the Mass-Radius relation with a simple numerical technique based on this view. If our interpretation is correct, early-type galaxies formed at high redshifts via primordial mergers of small subunits, and fixed their dimensions ab initio with little modifications in later times. Furthermore, most of them were formed before z = 2 - 1, thus ruling out the necessity for late mergers.
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http://arxiv.org/abs/1206.2532
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